I've got both games for 360 and played them each to death.
    I like the guitar for Rock Band much more than the GH one, playing the solos on the high frets without strumming is pretty fun, because you can drink while you're playing with one hand. However, both my drums and guitar are broken, and I've had to fix them several times myself.
    The best songs in Rock Band (Green Grass, High Tides, Foreplay/Longtime, Run to the Hills) are better than the best songs in GH3 (save for Cliffs of Dover) and the downloadable content each week is pretty awesome.
    It's only fun to play Rock Band with a group now, and even still you'd have to twist my arm, but any time someone is playing GH3, I've got to hop in a get a turn.
I can't see the rhythm game phase lasting too much longer, I think its played itself out.